The cell fragments known as platelets are disc-shaped, with an average diameter of about 3 μm and a thickness of roughly 1 μm. They play a crucial role in the body's vascular clotting system, which also involves plasma proteins, blood cells, and blood vessel tissues.

Platelets are continually replenished, circulating in the bloodstream for 9-12 days before being removed by phagocytes, primarily in the spleen. A microliter of circulating blood contains between 150,000 and 450,000 platelets, with an average concentration of 350,000/μL. Approximately one-third of the body's platelets are present in the spleen and other vascular organs at any given time instead of circulating in the bloodstream. These reserves can be mobilized during a circulatory crisis, such as severe bleeding.

Platelet production, or thrombocytopoiesis, occurs in the red bone marrow. Megakaryocytes, large cells with giant nuclei, are responsible for producing platelets. These cells manufacture various proteins, enzymes, and membranes during their development. Eventually, they release platelets into the bloodstream. A mature megakaryocyte can generate approximately 4,000 platelets before its nucleus is broken down and recycled by phagocytes.

Megakaryocyte activity and platelet formation are influenced by three substances: thrombopoietin (TPO), interleukin-6 (IL-6), and multi-CSF. Thrombopoietin, a hormone produced in the kidneys, stimulates platelet formation and the production of megakaryocytes. Interleukin-6 also stimulates platelet formation, while multi-CSF promotes the growth and formation of megakaryocytes, thereby increasing platelet production.

Platelets have several functions, including releasing chemicals that are vital in clotting. By releasing enzymes and other factors, platelets help initiate and control clotting. They can also form a temporary patch in the walls of damaged blood vessels by clumping together at the injury site, creating a platelet plug that can slow blood loss while clotting occurs. Finally, platelets contain filaments of actin and myosin, which can contract to reduce the size of the break in the vessel wall after a blood clot has formed.